Austenitic stainless steel and process therefor



1966 R B. G. YEO ETAL 3,284,250

AUSTENITIC STAINLESS STEEL AND PROCESS THEREFOR Filed Jan. 9, 1964HR.+2/2300F +CR+II2000F ALLOY f 7 United States Patent Ofitice 3,284,250AUSTENlTMZ STAINLESS STEEL AND PRUCESS THEREFGR Ralph B. G. Yeo,Westtield, N.J., and Tom E. Scott, Ames, Iowa, assignors to TheInternational Niche] (Iornpany, Inc, New York, N.Y., a corporation ofDelaware Filed Jan. 9, 1964, Ser. No. 336,808 8 Claims. (Cl. 148l2) Thepresent invention relates to austenitic stainless steels and, moreparticularly, to austenitic stainless steels of special composition andof the A151 300 series type which in the annealed condition arecharacterized by a combination of properties markedly superior to thosecharacteristic of known austenitic stainless steels.

Of the three principal classes of stainless steels, to Wit, themartensitic, ferritic and austenitic, the austenitic grades have by farfound the greatest commercial use as is evident from the fact thatpresent commercial produc tion of the austenitic grades is more thandouble the combined production of the martensitic and ferritic grades.This is not, in retrospect, surprising in view of the combinations ofproperties characteristic of the austenitic stainless steels, includingtheir high degree of resistance to corrosive environments, theirexcellent tensile strength levels at both normal and high temperatures,their established ability to be fabricated with relative case on acommercial scale, etc. These factors, among others, have led to theirwide acceptance and diversified utility and application.

In view of the fact that the austenitic stainless steels such as the18-8 type (18% chromium, 8% nickel) containing up to 2% manganese and upto 1% silicon have been known for about a half-century, it is rathersurprising to find that there is a dearth of literature with regard tothe specific problem of improving their yield strength in the annealedcondition While maintaining the high level of other properties, e.g.,ductility, corrosion resistance, etc. This seems particularlysignificant when it is considered that a substantial percentage of 18-8stainless is used in the annealed state. Actually, from the commercialviewpoint, the representative yield strength of such steels has remainedvirtually unchanged since their development. For example, in theauthoritative treatise Metals Handbook, the 1936 edition indicates atpage 379 that the yield strength of the 18-8 type of stainless steel(18% chromium, 8% nickel) was about 35,000 pounds per square inch(p.s.i.) in the annealed condition and exhibited an elongation value ofabout 55% to 60% (2 inch gage length). The 1961 edit-ion, page 414,reflects that a comparable 18-8 type, i.e., A181 302 or 304, hasapproximately the same yield strength and ductility (elongation) levelsin the annealed condition. This is not to say that very high yieldstrengths cannot be obtained with austenitic stainless. In thecold-worked condition, yield strengths above 200,000 psi. have beenobtained, but, as is well known, cold-worked austenitic stainless israther the antithesis of austenitic stainless in the annealed condition.

The continuous rise in the production of 18-8 austenitic stainless andthe rather concerted efforts to expand the applications and usesthereof, has stimulated the need for austenitic stainless steels capableof manifesting yield strengths in the annealed condition of a magnitudesubstantially superior to those exhibited 'by austenitic stainlesssteels presently available. For example, trailer tankers made ofaustenitic stainless have recently become more widely accepted. Thesetankers possess the virtues of disposing of, as a practical matter, theneed for maintenance and economically rovide for the transportation of avast variety of otherwise corrosive media as a result 3,284,250 PatentedNov. 8, 1966 of the resistance to corrosion afforded by austeniticstainless. However, it would be advantageous for such use to provide anaustenitic stainless steel of higher yield strength in the annealedcondition to thus afford greater resistance to external loads or stress.This aspect would also be economically attractive in enabling thinnersections of stainless steel to be used due to increased strength. Theproblem is intensified by the fact that other properties heretoforecharacteristic of such steels must not be detrimentally alfected. Thatis to say, the ductility, toughness, corrosion resistance, etc., forwhich the austenitic stain-less steels are so well known should not besacrificed at the expense of improved yield strength. Thus, for example,rather close adherence to the heretoifore prescribed compositionallimits for austenitic stainless :is necessary to preclude formation ofsecondary phases which would impair or adversely affect the mechanicaland/or chemical properties of such steels.

It has now been discovered that austenitic stainless steels of the A181300 series type but containing special amounts of columbium inconjunction with correlated amounts of other essential constituents arecapable of exhibiting yield strengths of 50,000 psi. to 60,000 psi. inthe annealed condition provided the common process annealing treatmentis eliminated. Such yield strength levels obtain without a concomitantdeleterious impairment of other properties.

The incorporation of columbium in austenitic stainless steels as such isnot new. It is believed that the use of columbium to overcome theproblem concerning intergranulalr corrosion was first proposed about1930 or shortly thereafter. Subsequent to the original development ofthe austenitic stainless steels, it was found that such steels wereafllicted with intergranular corrosive attack. More specifically, it wasfound that slowly cooling such steels through the temperature range ofabout 750 F. or 800 F. to 1600 F. (now commonly referred to assensitizing) resulted in chromium carbide precipitation at the grainboundaries. This resulted in a depletion of eiiective chromium and thuscreated an enviroment conducive to corrosive attack. Among the many waysadvanced to overcome this problem was the proposal, now well accepted,of stabilizing the austenitic stainless steels with various carbidestabilizers, one of which is columbium. Col-umbium, having a greaterafiinity for carbon than chromium, would combine with carbon to formcolumbium carbides} which were practically insoluble in the austenite atthe sensitizing temperature thereof. This preferential or selectivecolumbium carbide formation prevented or greatly minimized the tendencyfor chromium carbides to form at the grain boundaries. A certain ratioof columbium to carbon had to be observed and the ratio that has beengenerally adopted is that the amount of columbium must be at least tentimes the carbon content as is reflected by the standard columbiumstabilized grade of stainless steel, i.e., A181 347. The other carbidestabilizers act in a similar manner. Of course, as is well known,intergranular corrosion is also greatly minimized by maintaining thecarbon level below 0.03%, thus rendering the use of columbium or otherstabilizers unnecessary.

It is common commercial practice to subject austenitic stainless steelsduring processing to intermediate annealing treatments at temperaturesof about 1900 F. to 2100 F. The final annealing treatment is alsousually conducted over the range of 1900 F. to 2100 F. in commercialoperation. In addition, the use of a process anneal is commonly employedbetween hotand coldworking operations and is particularly utilized withregard to the production of austenitic stain-less in the form of sheet.Among other principles of the invention, we

up to can be tolerated.

have now found that by avoiding the process annealing Step an optimalyield strength is attained apart from the economic benefit-s realized.

It is an object of the present invention to provide austenitic stainlesssteels of improved yield strength in the annealed condition withoutadversely affecting the other properties, including ductility, of suchsteels.

Another object of the invention is to provide a process for achieving acombined high level of yield strength and ductility in austcniticstainless steels in the annealed condition.

Other objects and advantages will become apparent from the followingdescription taken in conjunction with the accompanying drawing .inwhich:

FIGURES 1 to 5 are reproductions of photomicrographs showing thestructure of various austenitic stainless steels after being subjectedto different heat treatments.

In accordance with the invention an optimum combination of properties,including yield strengths of atv least about 50,000 psi. (0.2% offset)together with ductilities of at least 50% (using standard ASTMspecimens) are obtained in the annealed condition with austeniticstainless steels having com-positions (based on weight percentage)within the following most advantageous ranges: at least 0.05%, 'e.g.,0.06%, and up to about 0.08% carbon, about 17.5% to about 19.5%chromium, about 8% to about 12% nickel, about 0.18% to about 0.28%columbium, up to about 2% manganese, up to about 1% silicon, up to about0.1% aluminum, up to 0.5% nitrogen and the balance essentially iron. Thesteels contemplated herein have a unique fine-grained structure, i.e.,an ASTM grain size of 11 to 14 or finer, and are substantially free ofcoarse carbide precipitates. This characteristic of an extremely finegrain structure is discussed more fully hereinafter.

To obtain yield strength levels of-50,000 p.s.i. in the annealedcondition it is most important that the aforediscussed process annealingtreatment, i.e., the anneal between the hot working and cold workingoperations at temperatures above about 1900 F., be avoided as will bemore fully illustrated herein. Further, the constituents of the alloycomposition must be balanced and correlated such that at the workingtemperature employed in processing, i.e., 2300 F. down to above 1900 F.,the presence of detrimental delta ferrite is avoided. If appreciableamounts of delta ferrite are present during the hot-working operation,delta ferrite will be retained in the final product and while itincreases yield strength, it substantially adversely affects ductilityand formability properties particularly when such properties aremeasured in the transverse direction. Most advantageously, the presenceof delta ferrite should be avoided although Improved yield strengthlevels in the annealed condition can also be attained with austeniticstainless steels within the following compositional ranges -and withoutdetrimentally affecting other properties: carbon in an amount more than0.03%, e.g., more than 0.04%, and up to 0.12%, at least 16% and up to20% chromium, about 6% to about 12% nickel, 0.15% to not more than 0.5columbium, up to 2% manganese, up to 2% silicon, up to 0.5 aluminum, upto 0.5% nitrogen and the balance essentially iron. Of course, thealloying constituents must be properly cor- (related to provide for anaustenitic structure.

In carrying the invention into practice, it is preferred to soak thesteel at a high temperature, such as 2250 F. to 2350" F., e.g., 2300 F.,before hot rolling and to then apply and continue the hot-rollingtreatment within the temperature range of 2300 F. down to above 1900 F.Temperatures as low as 1650 F. are commonly employed but it is mostadvantageous to hot work from a temperature of about 2300 F. down to2000 F. After this treatment, the steel has a hardness as low asannealed material and is suitable for subsequent cold rolling. If thefinishing temperature of the hot-working operation falls significantlybelow about 2000 F., e.g., below about 1950 F., a high temperatureprocess annealing treatment may otherwise be required at, say, 2300" F.But since it is desirable to eliminate the process anneal, it isbeneficial to maintain a minimum hotworking temperature of about 2000 F.

Subsequent to the hot-working treatment the steels are subjected to acold reduction operation for control of gage and surface finish and toassure the occurrence of recrystallization during the final annealtreatment (the only annealing treatment required in accordance with theinvention). The steel should be advantageously cold reduced at least 15%and up to about 70%. Cold reductions of less than about 15%, e.g., 5%,tend to cause excessive grain growth while reductions of over 70% resultin a steel which is too hard to further work without difficulty on acommercial basis. Cold reductions of 40% to 60% are most advantageouswith 50% being highly satisfactory. Where desired, intermediateannealing may be employed provided that the temperature range of 1900 F.to 2200 F. is avoided. If the cold-working operation or equivalent isomitted, it makes little difference at what temperature the final annealis conducted since yield strengths of at least 50,000 p.s.i. will not beobtained.

In accordance with the invention, it has been found that the heattreatment employed (after cold working) to achieve the annealedcondition is important. The most advantageous temperature range is 1750F. to 1850 F. and the steel should be held within this range for about 3hours to 0.4 hour, e.g., 1 hour at 1800 F. While the annealingtemperature can be as high as 2000 F., the holding time should be notgreater than 5 minutes and preferably not greater than 3 minutes,otherwise, adverse results can occur. This stems from the fact that inaccordance with the invention it is considered that the unusually highyields strengths result, inter alia, partially from the very fine grainsize, i.e., ASTM 11 to 14 or finer, and partially from dislocationtangles. High annealing temperatures, i.e., 2000 F., bring about acondition in which fine carbide particles go back into solution and thisis accentuated with long holding times, e.g., one-half hour at 2000 F.Thus, there is a loss of fine particles which would otherwise retardgrain boundary movement and this would be causative of grain growth.Also, the dislocation tangles tend to disappear or combine with eachother such that there is an interference with the strengtheningmechanism. On the other hand, the minimum annealing temperature must beabove 1600 F., e.g., 1700 F. and above, to assure the occurrence ofrecrystallization. However, at the lower annealing temperature longerholding times would be required and this contributes to prolongedprocessing. At 1750 F. the steels should be annealed for about 3 hours.For optimum results the annealing treatment should be in accordance withthe following formula:

45.3:(460-1-1") (log t+20) 10- where T is the temperature in degreesFahrenheit and t represents time in hours. This formula is derived fromthe Larson-Miller parameter equation. Thus, at 1900 F. the holding timeshould be approximately 9 to 10 minutes. A temperature range of 1725 F.to 1950 F. is also satisfactory and the steels should be held at suchtemperatures for about 4 hours to 0.4 hour, the period of the shortestholding time being used at the highest temperature.

For the purpose of giving those skilled in the art a betterunderstanding of the invention and/or a better appreciation of theadvantages of the invention, the following illustrative examples aregiven:

A series of stainless steels (Alloys 1 to 11 and A to E) was preparedhaving the compositions given in Table l.

3,284,250 a a For purposes of comparison the compositions for AISIAlloys Nos. 3 through 7 of Table II illustrate that yield 304 and 347 asset forth at page 409 of the Metals strengths of Well over 50,000p.s.i., e.g., 55,000 p.s.i.,

Handbook, 1961 edltron, are also g1ven in Table I. together with hlghductility (1.e., over 50% elongatlon TABLE I Alloy N o. 0, percent Ni,percent Or, percent Cb, percent Si, percent Mn, percent Fe,

percent 0.08 max 8-12 18*20 1.00 max 2.00 max Bnl. 0.08 max 9-l3 17-1910x min l.0t)1nax 0.07 9. 65 18. 73 N.A 0.39 0.07 9.60 19. 09 0.09."0.59 0.08 9. 60 18.86 0.16--- 0.57 0.07 9. 55 18. 81 0.26 0.50 0.07 9.60 19.01 0.28- 0.48.--- 0.07 9. 60 18. 60 0.34 0.5 0.07 9. 60 18. 940.46 0.45 0.022 9. 15 18. 15 N.A 0.39 0.032 9. 75 18. 45 0.08 0.40 0.0269. 75 18. 60 0.17 0.45 0.024 9. 75 18. 50 0.27. 0.40 0.06 10.65 18. 08N.A 0.63 0.071... n. 9.85 18.14 0.1 0.57"" 0.07- 10. 18. 68 0.25 0.660.07 9. 8 17. 56 0.51- 0.67.--- 0.07 10. 05 18. 62 0.85 0.63

1 Bal.=Balance essentially iron. I 2 x 0 min. Columbium exceeds carbonby a minimum ratio of 10 to 1. N.A. =Nonc added.

The steels 1 through 11 and A through B were prepared and over 70%reduction in area) can be obtained in in an air induction furnace, theingredients being melted austenitic stainless steels in the annealedcondition. Such in magnesia crucibles. Silicon-manganese was used forstrength levels represent an increase of about 50% or deoxidationpurposes with aluminum additions of less more as can be seen from acomparison of Alloys AISI than about 0.1% being made. Final deoxidizingaddi- 304 and 347 with Alloys Nos. 3 through 7. It will be tions weremade following slag removal and normal further noted that Alloys Athrough E (which were temperature adjustment. The steels were producedin given a process anneal) manifested significantly lower ingot form andhot rolled at temperatures of 2000 F. r yield strengths than Alloys Nos.3 through 7. Further, or 2300 F. after a liberal soak. The hot-rolledproduct the steels containing less than 0.03% carbon (Alloys Nos. was inthe form of A-inch round bars. Alloys Nos. 1 8, 10 and 11) allmanifested yield strengths much below through 11 were subjected to atreatment whi h com- 50,000 p.s.i. and such data illustrate that thesteels should ri ed old redu in the b about 45 d annealing containcarbon contents in excess of 0.03%. In addif abo t 1 ho t 1800 F, f ll db i li tion, Alloys Nos. 1 and A indicate that in the absence of Noinitial or process anneal was employed. Alloys A columbium it isimmaterial whether the steels are or are through E were treated in thesame manner except a not given a Process anneal treatmentprocess neal of1 ho t 2000 F, was employed prior A striking feature is the effect ofcolumbium in acto cold working. It should be noted that Alloys Nos.{30111211196 with invwti'on as pp to columbium- 2 through 7 and Bthrough E respond to the composition Containing A151 347 Which has arepresentative Yield of A181 304 except for the columbium content. Itstrength of y 35,000 P- in the annealed n nh ld l b t d h All N 3 10 d11 d No special product ratio of columbium content times the carbonContents below 003% and, thus are Outsida the carbon content isnecessary. The data further reflects Scope of the present invention asare Alloys 1, that for an optimum combination of properties, the 2 and 9(columbium was not added to Alloy 1 columbium content should not exceed0.3%.

and the amount present in Alloys 2 and 9 was Residual cold work was notresponsible for or causalow the minimum required in accordance with theinventive of tha markedly enhanced yield Strengths illustrated tion).The yield strength 0.2% offset), ductility (elonby data, m Table Todemonstrate this gation in percent using standard ASTM specimens) andauSteI.nt1c Stamless steel 9 the A181 304 type. but which percentEduction of area are given in Tab 1e IL contained 0.19% columbium (andalso contained 0.08%

carbon, 9.01% nickel, 18.27% chromium, 0.76% silicon TABLE H and 1.08%manganese) was hot rolled to strip. The

hot-rolled strip was fully recrystallized and had a Rock- Yi jd Epmg.Reduction Well B hardness of 75 to 80. The hardness level did Alloy eg3; g g 2 522 not fall further even after an annealing treatment of 1 psihour at a temperature as high as 2100 F. Specimens of the steel werecold reduced 45% without an initial 25,000 55 anneal and were annealedafter cold reduction (the only 55:28, 9g anneal) at 1800 F. The 0.2%offset yield strength $2,5 was 50,500 p.s.i. and the elongation was 55%.These 551300 55 65 results are in good agreement with Alloy No. 3 ofTable 2 II which contained 0.16% columbium.

551200 51 While the theory which would perhaps explain the $8 $2.8mechanism involved is not completely understood, it is 371900 5 deemedthat columbium exerts a most unusual and proggg 23.? nounced influencein retarding grain growth when used 1 1n percentages in accordance withthe invention, al 2%. $88 though solid solution hardening mechanismsmight be in- 4O:]00 volved This is considered quite opposite inconjunction with the elimination of the process annealing treatment Thisostensible phenomenon is more eifectively illus- Mctals Handbook, 161cd., page 414.

trated by reference to FIGURES l to wherein there is shown the grainsize of Alloys Nos. 1, 4 and 6, respectively, in various treatedconditions. In FIG. 1 (250 magnification) there are illustratedphotomicrograph reproductions of the three steels in hot hot-rolledcondition (hot rolled at 2300 F.). FIG. 2 (100 magnification) depictsthe grain size of each of the steels after a process anneal at 2300 F.for 2 hours following the hot-rolling operation. It will be noted thatthe process anneal completely eliminated all observable difierence amongthe three steels. FIGURES 3 and 4 (both at 250 magnification) representthe microstructure obtained after a final anneal, i.e., the alloys werehot rolled, process annealed at 2300 F. for 2 hours, cold rolled (areduction in area of about 45%) and given a final anneal at 1800 F.(FIG. 3) and 2000 F. (FIG. 4) for 1 hour. FIGURE 5 (250 magnification)depicts the extremely fine grain structure (finer than ASTM size 14)obtained for Alloys Nos. 4 and 6 (alloys within the invention) in theabsence of the process annealing treatment. The steels (FIG- URE 5) werehot rolled, cold rolled (45% reduction in area) and annealed at 1800 F.for 1 hour. Thus, austenitic stainless steels within the invention areuniquely characterized by a very fine grain structure, i.e., an ASTMgrain size of 11 or finer, e.g., ASTM grain size 14 or finer, in theabsence of a process anneal. This is thought surprising when consideredin the light of the grain size heretofore given for the well establishedcolumbiumcontaining austenitic stainless AISI 347. For example, it hasbeen reported that the AISI 347 austenitic steel of the followingcomposition has a grain size of about 6 or 7.

When hot rolled, pickled and then annealed at 1900 F., 2000 F. and 2100F. this steel had a grain size of 8, 7 and 6, respectively. When hotrolled, annealed at 1900 F., 2000 F. and 2100 F., respectively,de-scaled, cold rolled and final annealed at the same temperatures asthe initial anneal, this steel had a grain size of 7, 7 and 6,respectively. This compares quite unfavorably with ASTM grain sizes of11 to 14 achieved in accordance with the invention. It is consideredthat this conflicting behavior is somewhat attributable to the fact thatcolumbium in amounts say above 0.5%, result in carbide agglomerateswhich paralyze the reactions taking place to the extent that graingrowth is substantially uninhibited. We have found that high columbiumcontents should be avoided and are quite unnecessary. An alloy stainlesssteel containing 0.07% carbon, 10.05% nickel, 0.63% silicon, 1.2%manganese, 18.62% chromium and 0.85% columbium when hot rolled, coldreduced by 45% and annealed at 1800 F. for 1 hour had an ASTM grain sizeof about 8. Except for the columbium content, this steel would have beenwithin the invention.

Whatever be the theoretical explanation, the austenitic stainless steelsin accordance with the invention are characterized by extremely finegrain sizes, i.e., ASTM grain sizes of at least 11, e.g., 12 to 14. Thisfine grain size affords many other advantages, such as improveddeepdrawing characteristics and machinability. It should be mentionedthat grain size as referred to herein, was determined with the StandardASTM Grain Size Chart at a magnification of 500x. This is higher thanthe 100 magnification normally used because of the extreme fineness ofthe grains of the steels within the invention. Correction for highermagnification was made by using the ASTM correction formula,

Q=6.64 log in S where Q is the correction number to be added to the ASTMGrain Size Number observed at the higher magnification, M is the highmagnification, i.e., 500x, and M equals x.

The austenitic stainless steels of the invention can, of course, be usedin all applications for which such steels are presently used. However,by virtue of the increased yield strength coupled with good ductility,etc., characteristic of the steels, an expanded field of use is openedparticularly where higher strength-to-weight ratios are necessary oradvantageous, e.g., transportation tanks carrying various media,including corrosives.

As those skilled in the art will readily appreciate, balance or balanceessentially as used herein in referring to the iron content of thesteels does not exclude the presence of other elements commonly presentas incidental elements, eg., deoxidizing and cleansing elements, andimpurities ordinarily associated therewith in amounts which do notadversely affect the basic characteristics of the steels. For example,up to 3% molybdenum can be present in the steels. The term austenitic asused herein means that the structure of the alloys at room temperatureis substantially or completely of an austenite matrix although up to notmore than 10%, and most advantageously not more than 5%, of otherphases, e.g., ferrite, can be present.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. For example, the present invention is applicable to theaustenitic stainless steels of the AISI 201 and 202 types, i.e., thenickel content can be as low as 3% and the manganese content can be ashigh as 10%, the remainder of the composition being within the limitsset forth before herein. Of course, the nickel and manganese contents incombination with the other elements would have to be correlated toinsure an austenitic structure. In such instances the yield strength ofthe AISI 201 and 202 types is increased to above 70,000 p.s.i., e.g.,75,000

to 80,000 p.s.i. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

We claim:

1. A process for improving the yield strengths of and achieving finergrain sizes in austenitic stainless steels consisting of about 0.06% toabout 0.08% carbon, about 8% to about 12% nickel, about 17.5% to about19.5% chromium, about 0.18% to about 0.28% columbium, up to 1% silicon,up to 2% manganese, up to 0.1% aluminum, up to 0.3% nitrogen and thebalance essentially iron which comprises hot working the steels withinthe temperature range of about 2300 F. to about 2000 F., cold workingthe steel without a prior process annealing treatment -to obtain areduction in area of 40% to 60% and thereafter recrystallizing the steelby subjecting it to an annealing treatment within the temperature rangeof about 1750" F. to about 1850 F. in accordance with the followingformula:

wherein T represents temperature in degrees Fahrenheit and t is time inhours.

2. A process for improving the yield strengths of and achieving finergrain sizes in austenitic stainless steels consisting of carbon in anamount above 0.03% and up to about 0.12%, about 6% to about 12% nickel,about 16% to about 20% chromium, about 0.15% to less than about 0.3%columbium, up to 2% silicon, up to 2% maganese, up to 0.5% aluminum, upto 0.5 nitrogen and the balance essentially iron which comprises hotworking the steels within the temperature range of about 2000 F. and upto 2300 F., cold working the steel without a prior P SS annealingtreatment to obtain a reduction in area of at least 15% and thereaftersubstantially recrystallizing the steel by subjecting it to an annealingtreatment within the temperature range of about 1725 F. to 1950 F. for aperiod of about 4 hours to about 0.4 hour, the period of the shortestholding time being employed at the highest temperature.

3. The process as set forth in claim 2 wherein the car bon content ispresent in an amount of at least 0.04%.

4. The process as set forth in claim 2 wherein the carbon content is atleast 0.05% and the columbium content is at least 0.18%.

5. A process for improving the yield strengths of and achieving finergrain sizes in austentic stainless steels consisting of about 0.05 toabout 0.12% carbon, about 6% to about 12% nickel, about 16% to about 20%chromium, 0.15% to not more than 0.5% columbium, up to 2% silicon, up to2% manganese, up to 0.5 aluminum, up to 0.5% nitrogen and the balanceessentially iron which comprises hot working the steel within thetemperature range of above 1900 F. and up to 2300 F., cold Working thesteel without a prior process annealing treatment to obtain a reductionin area of at least 15 and thereafter substantially recrystallizing thesteel by subjecting it to an annealing treatment within the temperaturerange of above about 1700 F. and up to not more than about 2000" F.

6. An austenitic stainless steel in the annealed condition consisting ofat least 0.05% and up to 0.12% carbon, about 3% to about 12% nickel,about 16% to about 20% chromium, about 0.16% to not more than 0.3%columbium, up to 1% silicon, up to manganese, up to 0.1% aluminum, up to0.3% nitrogen, up to 3% molybdenum and the balance essentially iron,said steel being characterized by a yield strength of at least 50,000p.s.i., an elongation of at least and a grain size of at least aboutASTM No. 12 when hot worked at a temperature of about 2000 F. to about2300 F. followed by cold working to obtain a reduction in area of atleast 15% and then substantially recrystallized by subjecting it to anannealing treatment within the temperature range of over about 1700 F.to not more than 2000" F.

7. An austenitic stainless steel consisting essentially of carbon in anamount above 0.03% and up to 0.12%, about 3% to about 12% nickel, about16% to about 20% chromium, about 0.15% to about 0.3% columbium, up to 2%silicon, up to 10% manganese, up to 0.5% aluminum, up to 0.5% nitrogen,up to 3% molybdenum, and the balance essentially iron, said steel havinga yield strength of at least 50,000 p.s.i., an elongation of at least50% and a grain size fineness of at least about ASTM No. 12.

8. The stainless steel as set forth in claim 7 and containing 0.04% to0.12% carbon, 6% to 12% nickel, 16% to 20% chromium, 0.16% to 0.28%columbiurn, up to 2% silicon, up to 2% manganese, up to 0.1% aluminum,up to 0.3% nitrogen, and the balance essentially iron.

References Cited by the Examiner B, Ca, Cb, and Zr, in Iron and Steel,Grange, Shortsleeve and Hilty, Binder, Motock and Ofienhauer, 1957, JohnWiley and Sons, Inc., page 279.

DAVID L. RECK, Primary Examiner.

H. F. SAITO, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,284,250 November 8, 1966 Ralph B. G. Yeo et a1.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 6, line 73, for "volved This is considered quite opposite" readvolved. This is considered quite apposite line 74, after "treatment"insert a period; column 7, line 5, for "hot", first occurrence, read theline 49, for "amounts say" read amounts, say column 8, line 17, for"eg." read e.g. line 70, for "maganese" read manganese Signed and sealedthis 12th day of September 1967.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner ofPatents

1. A PROCESS FOR IMPROVING THE YIELD STRENGTHS OF AND ACHIEVING FINERGRAIN SIZES IN AUSTENITIC STAINLESS STEELS CONSISTING OF ABOUT 0.06% TOABOUT 0.08% CARBON, ABOUT 8% TO ABOUT 12% NICKEL, ABOUT 17.5% TO ABOUT19.5% CHROMIUM, ABOUT 0.18% TO ABOUT 0.28% COLUMBIUM, UP TO 1% SILICON,UP TO 2% MANGANESE, UP TO 0.1% ALUMINUM UP TO 0.3% NITROGEN AND THEBALANCE ESSENTIALLY IRON WHICH COMPRISES HOT WORKING THE STEELS WITHINTHE TEMPERATURE RANGE OF ABOUT 2300* F. TO ABOUT 2000* F., COLD WORKINGTHE STEEL WITHOUT A PRIOR PROCESS ANNEALING TREATMENT TO OBTAIN AREDUCTION IN AREA OF 40% TO 60% AND THEREAFTER RECRYSTALLIZING THE STEELBY SUBJECTING IT TO AN ANNEALING TREATMENT WITHIN THE TEMPERATURE RANGEOF ABOUT 1750* F. TO ABOUT 1850* F. IN ACCORDANCE WITH THE FOLLOWINGFORMULA: 45.3=(460+T) (LOG T+20) X 10 **3